Synthetic rubbers produced from, for example, butadiene homopolymers and statistical butadiene/styrene copolymers employing organolithium catalysts have become increasingly popular in technical manufacturing processes, especially in the production of passenger car tire treads.
During the course of this development, it has been found that an important governing factor for the processing properties of the synthetic rubber, in addition to its long-chain branching, is its molecular weight distribution.
A broadening of the molecular weight distribution results, for example, in improved roll adhesion, shorter mixing time, lower mixture viscosity and lower extrusion temperature.
To attain such an improvement in the processing characteristics of the rubber, polymerization techniques have been developed which lead to polymers having a broadened molecular weight distribution. Very many of these processes, e.g., those wherein branching reagents are utilized, do effect a broadening of the molecular weight distribution, but lead to an increase in the long-chain branching.
However, it is definitely desirable to increase the breadth of the molecular weight distribution alone, with the long-chain brancing remaining unchanged, so that the aforementioned processing properties of the rubber can be improved in a precisely controlled fashion.
Thus, a process for the polymerization of 1,3-dienes is known from Unexamined Laid-Open German Pat. No. DOS 1,520,854 wherein, after a degree of polymerization of about 70-80% conversion has been attained, the reaction is terminated and catalyst is once more introduced, thus making it possible to manufacture a high-molecular proportion and a low-molecular proportion and concomitantly attaining a broadening of the molecular weight distribution.
Furthermore, a process for the polymerization of 1,3-butadiene is known from U.S. Pat. No. 3,382,224, wherein solvent is added during the course of the polymerization for dilution purposes. In this process, the impurities contained in the solvent cause the growing polymer chains to be partially broken off, whereby a broadening of the molecular weight distribution in the polymer is obtained.
A process for the polymerization of 1,3-butadiene and for the copolymerization of 1,3-butadiene with styrene, respectively, is disclosed in DOS German Pat. No. 2,125,694, wherein a partial stream is branched off from the polymerization, and this stream is reintroduced into the reactor. Here again, a broadening of the molecular weight distribution of the polymer is achieved.
In the prior art methods described above, the processing properties of the rubber are improved by broadening the molecular weight distribution. However, all known processes have the feature in common that at least two liquid partial streams of differing viscosity must be mixed together during the course of the polymerization, entailing a considerable expenditure in energy. Moreover, in all such conventional processes, the course of the polymerization must be interfered with. Since the Li-catalyst based polymerization system reacts with extreme sensitivity to impurities, this means a difficult and often uncontrollable progression of the reaction.
Accordingly, it is an object of this invention to provide a method for producing polymers having a broad molecular weight distribution which lacks the prior art disadvantages for achieving this objective. Other objects will be apparent to those skilled in the art.